Evidence has been obtained to indicate that the myelin proteolipid protein as well as an unidentified non-myelin proteolipid are associated with both the jimpy and quaking defects. Preliminary evidence has also been gathered which indicates that the jimpy brain contains detectable levels of at least one of the myelin basic proteins but that the levels of both basic proteins as well as the proteolipid protein are significantly reduced in isolated myelin from the mutant. Clear differences have also been observed in the lipid composition of large and small myelin.